1. Field of the Invention
The present application relates to a pressure-sensitive adhesive composition for an optical film, a polarizing plate and a liquid crystal display.
2. Discussion of Related Art
A liquid crystal display (LCD) usually includes a liquid crystal panel containing a liquid crystal component injected between two transparent substrates and an optical film. As an optical film, there is a polarizing film, a retardation film or a brightness enhancing film. To laminate these optical films or attach an optical film to an adherent such as a liquid crystal panel, a pressure-sensitive adhesive for an optical film is generally used.
A pressure-sensitive adhesive may use an acrylic polymer, rubber, a urethane resin, a silicon resin or ethylene vinyl acetate (EVA) resin, and as a pressure-sensitive adhesive for an optical film, particularly, a polarizing plate, a pressure-sensitive adhesive including an acrylic polymer which has excellent transparency, and high resistance to oxidation or yellowing is generally used.
Such a pressure-sensitive adhesive is prepared by coating a pressure-sensitive coating solution including an acrylic polymer and a crosslinking agent, that is, a pressure-sensitive adhesive composition, and then curing the composition. Meanwhile, to ensure durability and cohesive strength required for the pressure-sensitive adhesive particularly used for a polarizing plate, an acrylic polymer having a weight average molecular weight of 1,500,000 to 2,000,000 is usually included in the pressure sensitive adhesive composition.
The density of such a pressure-sensitive adhesive composition including a polymer having a large weight average molecular weight increases as the coating solid content included therein is set higher. Accordingly, when a large amount of solvent is input to coat a pressure-sensitive adhesive solution, coating productivity and coating uniformity are significantly decreased.
However, when the weight average molecular weight of the polymer is decreased to solve the above-mentioned problem, durability and re-workability of the pressure-sensitive adhesive are drastically decreased.
A method of preparing a pressure-sensitive adhesive by repeating a coating process will be used to achieve the desired thickness, but the coating solid content is set low. However, the method has problems of decreased productivity with respect to production cost, and difficulty in precise control of the thickness of the pressure-sensitive adhesive.
Japanese Patent Laid-Open No. 2011-057794 (Reference 1) discloses an attempt to increase the coating solid content and satisfy durability by blending multifunctional isocyanate and a radical initiator with an acrylic polymer having an average molecular weight of 500,000 to 1,000,000. However, it is necessary to increase drying temperature or drying time after coating in order to prevent residue of the radical initiator from forming.
Japanese Patent Laid-Open No. 2004-091500 (Reference 2) discloses an attempt to add a carboxyl, amide or amino group to an acrylic polymer with a hydroxyl group, which has a weight average molecular weight of 500,000 to 2,000,000, and adjust gel content after being crosslinked at 1 to 50%. However, a pressure-sensitive adhesive prepared by an above-mentioned method is difficult to be used in practice due to considerably low re-workability and great change with the passage of time.